RF communications systems typically communicate using at least one of three different modes of operation. The first mode, called simplex, is a one-way mode of operation, in which a transmitter from one location transmits data to a receiver at another location. For example, a broadcast radio station transmits data one-way to radios. The second mode, called half duplex, is a two-way mode of operation, in which a first transceiver communicates with a second transceiver; however, only one transceiver transmits at a time. Therefore, the transmitter and receiver in a transceiver do not operate simultaneously. For example, certain telemetry systems operate in a send-then-wait-for-reply manner. The third mode, called full duplex, is a simultaneous two-way mode of operation, in which a first transceiver communicates with a second transceiver, and both transceivers may transmit simultaneously; therefore, the transmitter and receiver in a transceiver must be capable of operating simultaneously. In a full duplex transceiver, signals from the transmitter must not interfere with signals received by the receiver; therefore, transmitted signals are at transmit frequencies that are different from received signals, which are at receive frequencies.
The difference between a transmit frequency and a receive frequency is called a duplex frequency. For example, certain cellular telephone systems, such as certain third generation (3G), fourth generation (4G), or later, may operate using a full duplex mode of operation. Full duplex transceivers using a single antenna often use a duplexer to couple the transmitter and receiver to the single antenna. A duplexer enables simultaneous transmission and reception of RF signals by providing a transmit passband that does not overlap with a receive passband, which prevents interference between transmit and receive signals. The non-overlapping area is also known as a guard band. However, since the transmit frequency and the receive frequency must typically both fall within a single RF communications band, the guard band may be fairly narrow. Additionally, some communications protocols, such as those associated with specific Universal Mobile Telecommunications System (UMTS) bands have guard bands that are fairly narrow. Therefore, the transmit and receiver passbands may need to be narrow with steep roll-offs. As a result, certain acoustic technologies, such as surface acoustic wave (SAW) or bulk acoustic wave (BAW) technologies may be used in bandpass filters, such as SAW filters or BAW filters, to provide narrow and steep passbands. Such bandpass filters are often used in duplexers.
As wireless technology evolves, the number and variations of wireless communications protocols increase and may encompass multiple operating modes, multiple frequency bands, and various transmit power levels. As a result, wireless communications products may need to provide support for many such protocols and frequency bands. Tens of frequency bands may need to be supported, such that each frequency band is associated with a dedicated duplexer. As such, many duplexers may be needed to support all of the frequency bands.
In this regard, FIG. 1 shows a multi-band front-end 10 according to the prior art. The multi-band front-end 10 includes an antenna 12, a first single-band duplexer 14, a second single-band duplexer 16, and up to and including an NTH single-band duplexer 18. Antenna switching circuitry is coupled between the antenna 12 and the single-band duplexers 14, 16, 18. The first single-band duplexer 14 receives a first transmit signal TX1 via a transmit port TXP and forwards the first transmit signal TX1 to the antenna 12 via a common port CP. Further, the first single-band duplexer 14 receives a first receive signal RX1 via the common port CP and forwards the first receive signal RX1 to first receive circuitry (not shown) via a receive port RXP. The second single-band duplexer 16 receives and forwards a second transmit signal TX2 to the antenna 12 and receives and forwards a second receive signal RX2 to second receive circuitry (not shown). The NTH single-band duplexer 18 receives and forwards an NTH transmit signal TXN to the antenna 12 and receives and forwards an NTH receive signal RXN to NTH receive circuitry (not shown).
FIG. 2 shows details of the first single-band duplexer 14 illustrated in FIG. 1 according to the prior art. The first single-band duplexer 14 includes a first series transmit SAW resonator 22, a second series transmit SAW resonator 24, a third series transmit SAW resonator 26, a fourth series transmit SAW resonator 28, and a fifth series transmit SAW resonator 30 coupled in series between the common port CP and the transmit port TXP. The first single-band duplexer 14 further includes a first shunt transmit SAW resonator 32 coupled between the junction of the first series transmit SAW resonator 22 and the second series transmit SAW resonator 24 and ground, a second shunt transmit SAW resonator 34 coupled between the junction of the second series transmit SAW resonator 24 and the third series transmit SAW resonator 26 and ground, a third shunt transmit SAW resonator 36 coupled between the junction of the third series transmit SAW resonator 26 and the fourth series transmit SAW resonator 28 and ground, and a fourth shunt transmit SAW resonator 38 coupled between the junction of the fourth series transmit SAW resonator 28 and the fifth series transmit SAW resonator 30 and ground.
Additionally, the first single-band duplexer 14 includes a first series receive SAW resonator 40, a second series receive SAW resonator 42, a third series receive SAW resonator 44, a fourth series receive SAW resonator 46, and a fifth series receive SAW resonator 48 coupled in series between the common port CP and the receive port RXP. The first single-band duplexer 14 further includes a first shunt receive SAW resonator 50 coupled between the junction of the first series receive SAW resonator 40 and the second series receive SAW resonator 42 and ground, a second shunt receive SAW resonator 52 coupled between the junction of the second series receive SAW resonator 42 and the third series receive SAW resonator 44 and ground, a third shunt receive SAW resonator 54 coupled between the junction of the third series receive SAW resonator 44 and the fourth series receive SAW resonator 46 and ground, and a fourth shunt receive SAW resonator 56 coupled between the junction of the fourth series receive SAW resonator 46 and the fifth series receive SAW resonator 48 and ground. The large number of SAW resonators 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56 illustrated in FIG. 2 may be needed to provide the narrow with steep roll-offs of the transmit and receiver passbands of the first single-band duplexer 14. However, SAW resonators tend to be relatively expensive and large. Thus, there is a need for duplexer functionality using multiple frequency bands with reduced size and cost.